Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.


Supplementary Figures
Supplementary Fig. Note: In terms of the structural configuration, water molecules in the typical liquid phase interact with adjacent molecules to form six-, five-or four-member ring structures.As the temperature increases, the interface environment undergoes a transition into a more disordered hydrogen bonding network.In the case of 500 K (coexist), the density of water molecules is already lower than that of the typical liquid water.Particularly under the condition depicted in Fig. 1f of the main text, the volume occupied by water molecules has almost doubled, resulting in a significant decrease in water density of 48%.Supplementary Fig. 1 reveals the numbers and lengths of hydrogen bonds as a function of the distance from the interface in six cases from the statistical analysis.As the volume increases, the average number of H-bonds per water molecule gradually decreases.Particularly, at the first and second water layers of the liquid/catalyst interface, there is a significant reduction in the number of H-bonds at 500 K (coexist).This demonstrates that the hydrogen bond network is destroyed to some extent once the liquid-gas transition begins to occur.Furthermore, as the H-bonds are disrupted, the average length of the hydrogen bonds also increases (Supplementary Fig. 1b).

Supplementary Fig. 2 | Statistical analysis of the MD simulations under different conditions.
The left shows the coverage of surface H2O molecules adsorbed at Ti5c site evolving with simulation time, while the right is the histogram of the resulting probability distribution.

Notes:
The statistical analysis of simulation with coverage of surface water molecules at different conditions was obtained from the last ~4 ps duration of each AIMD.Fig. 1a shows a structure of the liquid water/rutile TiO2(110) interface under standard conditions with an experimental water density of 1g/ml.It can be seen that water molecules close to the TiO2(110) surface bind to the Ti5c sites with the oxygen atoms of the water molecules in the first layer at ~2 Å from the surface.We determine the Ti-Owater bond length 2.32 Å as the chemisorbed bond length (equal to the length at the gas condition) and count the coverage changing with simulation time.As Supplementary Fig. 2 shows, at 298K (l), although the water molecules adsorb/desorb dynamically on/from the surface in the AIMD simulations, the coverage of surface water molecules is always approximately in the range of 0.75 ML (four molecules per surface TiO2(110) equal to 1 ML/surface coverage).When the temperature is increased but still below the phase transition temperature, the interfacial water network distribution is slightly affected by the temperature.At 373 K (l), the first layer water molecules are more likely to desorb from the surface, resulting in the water coverage being in the range of 0.5 ML to 0.75 ML.As for the 500 K (l), the coverage is dominant between 0.5 ML and 0.75 ML, but the probability of 0.25 ML has increased.For comparison, the vaporization process of liquid water under a high-pressure condition at 500 K was simulated (Fig. 1f).As the height of the total water layer increases, some water molecules in the first layer are more likely to desorb from the surface, resulting in the water coverage being in the range of 0.25 ML to 0.75 ML, where the 0.5 ML becomes the main coverage.
Supplementary Fig. 4 | The logarithmic plots of the OER rates at different water/TiO2(110) interfaces as a function of hole concentration.

Note:
The hole-electron recombination and the intensity of light irradiation are important for the overall photoactivity since they directly affect the hole concentration on the surface.In Supplementary Fig. 4, we highlight that the concentration of holes reaching the surface impacts significantly on the OER rate.Specifically, we investigated how the hole concentrations affect the OER rate under four typical conditions (298 K(l), 373 K(l), 500 K(l) and 500 K(coexist)) with the hole concentration ranging from 10 -10 to 1 ML, as illustrated in Supplementary Fig. 4. It can see the following: (i) the OER rates increase significantly from the very low concentration of surface-reaching hole and reach a plateau at a certain threshold value of the hole concentration.The threshold values become gradually smaller from 298 K(l)→373 K(l)→500 K(l)→ 500 K(coexist), indicating the relatively small dependence of hole-concentration on the liquid-vapor coexisting interface environments.(ii) Comparison with the other three conditions, it consistently exhibits better OER activity under the 500 K (coexist) condition with varying hole concentration, aligning with the corresponding activity trend described in Fig. 1h of the main text.
Here we take the case of 500 K(coexist) as an example to further illustrate the effect of hole concentration.When the hole concentration is in the range from 10 -10 to 10 -4 ML, an increase in hole concentration leads to a higher rate of OER.When the hole concentration exceeds 10 -4 ML, the OER rate reaches a plateau and the rate-determining step shifts, because at a higher hole concentration, the formation of • OHt via trapping holes at the terminal OHt− becomes easier and is no longer the rate-determining step.Moreover, the presence of micro-bubbles under the liquid-vapor coexisting conditions slows down the water dissociation reaction and make it earlier to be the rate-determining step (relative to the 298 K(l), 373 K(l) and 500 K(l) conditions).In this case, increasing the concentration of surface-reaching holes cannot further enhance the reaction rate.Note: It is worth mentioning that the stability of the hydrophobic substance in the in situ photocatalytic condition is an important issue for the realistic application.Here we use the hexafluoroacetone as a proof of concept to demonstrate the effect of hydrophobic microenvironment on modulating the photocatalytic OER, and some basic reasons using hexafluoroacetone are as follows: (i) it contains -CF3 groups, which has strong hydrophobic property; (ii) it has a C=O group, which allows it to adsorb onto the catalyst surface; moreover, its adsorption energy is relative lower than that of water molecule, and it will not excessively occupy the reaction sites.(iii) it is thermodynamically stable and does not chemically react with water molecules; for example, the enthalpy change of the reaction: H2O + (CF3)2C=O →(CF3)2CHO • + • OH, is strongly endothermic (>> 2 eV); (iv) regarding the photoexcitation effect, we evaluated the ability of C3F6O itself to trap holes, and we found that C3F6O has a significantly weak hole trapping capability (HTC) of about -0.24 eV, which is much weaker than that of OH − (-0.91 eV).As shown in the density of states (DOS) in Supplementary Fig. 8a, the highest occupied state of O in C3F6O is much lower, as low as -1.30eV, compared to the O of OH − (located at -0.5 eV).Therefore, C3F6O itself is less likely to be photoexcited under photocatalytic condition, making it difficult to decompose.

Supplementary
Further, we examined the possibility of oxidative conversion of hexafluoroacetone assisted by the surface OH • radicals to check the stability of hexafluoroacetone under the photocatalytic reaction condition, as illustrated in Supplementary Fig. 8c.Specifically, as (CF3)2C=O itself cannot not be self-activated via trapping hole (as shown in Supplementary Fig. 8a), the coupling of (CF3)2C=O with OH • radical ((CF3)2C=O * + * OH • → (CF3)2C(OH)O * , i.e., process 1→3) as the prerequisite reaction to activate (CF3)2C=O was calculated, including the possible dehydrogenation conversion of (CF3)2C(OH)O * to the carboxylate species * (CF3)2COO − .As shown in Supplementary Fig. 8c, the whole process (1→4) is exothermic but gives an effective barrier as high as 1.01 eV; this indicates that hexafluoroacetone could be oxidized by OH • radical in thermodynamics, but kinetically unfavorable at typical room temperature.In other words, hexafluoroacetone should be relatively stable, although the long-term stability could be an issue.We emphasize that it is the importance to consider the long-term stability when selecting the other hydrophobic substance for realistic application in further studies.

4-layer p(4×1)
surface with the experimental data reported by Wang et al. 5 , demonstrating the agreement between the calculated results (Ea=0.37 eV and ∆H=0.16eV) and experimental data (Ea=0.36eV and ∆H=0.035eV).This alignment underscores the reliability of our chosen model in capturing the energetics of this process.
Thirdly, one of the focuses in this current work is on accurately simulating the aqueous state at different conditions and obtaining the reliable reaction energies.According to the reference 6 , the water density distributions are very similar for the different slabs from 4 to 16 layers and overlap each other almost completely.This result further confirms that the 4-layer structure can be used to simulate a realistic solid-liquid interface system.interface water molecule), that is =l1-l2 as illustrated in Supplementary Fig. 11a.As shown in Supplementary Fig. 11b, the  value is stretched gradually from -0.65 to 1.0 Å during the H2O deprotonation process.For each fixed , we performed long-time MD simulation in NVT ensemble (T =298 K) until quasi-equilibrium state was achieved.All the interatomic forces along the reaction coordinate, which corresponds to the free energy gradients, can be obtained statistically.Then the free energy change can be obtained by integrating these free energy gradients.
Overall, the constrained MD is an accurate but very time-consuming method to calculate the complex OER network at the water/TiO2(110) interface.Alternatively, we developed the MPA-MD method to deal with the aqueous systems, and the reaction energetics were thoroughly tested in our previous work, including the reaction barriers, to verify the reliability of the MPA-MD method 2 .As shown in Supplementary Table 7, five kinds of aqueous interface reactions were compared between MPA-MD and the state-of-the-art constrained MD method.The MPA-MD method gives very similar results to the constrained MD method in all cases.In particular, for the H2O deprotonation reaction, both methods give the comparable barriers (0.50 versus 0.55 eV; see Supplementary Fig. 11c), demonstrating the feasibility of MPA-MD method in studying the OER mechanism at the water/TiO2(110) interface.
Secondly, the slow-growth based AIMD method is another effective way to calculate the free energy profile, which has been successfully used to capture the varying H-bonding networks during the aqueous reaction [11][12][13][14] .Compared to the common constrained MD method, the reaction coordinate () in the slow-growth method is changed linearly from state (1) to state (2) with a constant and very small transformation velocity  ̇.This is the biggest difference from the common constrained MD approach.The resulting free energy difference needed to perform a transformation from state (1) to state (2) can be computed as: where F is the free energy calculated at the coordinate  which evolves with t, and ∂/∂ is calculated along the MD trajectory by the SHAKE algorithm 15 .Overall, the slow-growth method has a relatively lower computational cost at a similar (or slightly worse) accuracy than the common constrained MD method, which was therefore also used to quickly determine the reaction coordinate prior to the common constrained MD method.Specifically, for the H2O deprotonation process, =l1-l2 was similarly chosen as the collective variable (CV) as shown in Supplementary Fig. 11d, and a very small value ∂ of 0.0005 Å was used in practice for each MD step; It is worth mentioning that the shorter step size for the describing slow-growth process along the reaction coordinate was also tested, which verified the validity of the ∂ value.Notably, the slow growth approach is available in the VASP code, and we used the standard exponentially weighted moving average (EWMA) method to process the average value of ∂/∂.An example of the raw output data from VASP and the integrated free energy profile is demonstrated in Supplementary Fig. 11d.The barrier of H2O deprotonation process with this approach is 0.52 eV, which is close to the value obtained from MPA-MD method (0.50 eV).

Notes:
The OER process starts from the water adsorbing on the Ti-row sites of TiO2(110) (step 1), and the H2Oad deprotonates in solution to form a Zundel-like (H5O2 + ) structure and produce the OHt − at the Ti-row site (step 2).Next, the generated OHt − could be further oxidized into • OHt via a hole trapping (step 3).Furthermore, the • OHt deprotonates, yielding the Ot − radical on the Ti-row site in a similar proton transfer mode with the H2Oad (step 4).The newly produced Ot − radical could couple with another adjacent Ot − radical on the Ti-row site, generating O2 2− (step 5).In this case, the O2 evolutes via the trapping of two successive holes to oxidize O2 2− (step 6 and 7).
It is interesting to find that the interfacial environment also remarkably influences the enthalpy change of the water dissociation process.Specifically, the enthalpy change of water dissociation at 298 K (l) is ~0.24 eV, suggesting that the dissociated water is more likely to reverse to H2O molecule rather than further oxidized.While at the states of 373 K (l) and 500 K (l), the enthalpy changes are reduced to ~-0.25 eV and ~-0.48 eV, respectively.It is noteworthy that the enthalpy change of deprotonation under the 500 K (coexist) condition is -0.54 eV, far surpassing the values of other interfaces, implying that the temperature can regulate the deprotonation processes by changing the interface environments.The enthalpy change of this step at 500 K (coexist) remarkably facilitates the forward reaction.This suggests that the appropriate coexisting environment of liquid and vapor can increase the driving force of the forwarding reaction by decreasing the final state energy (more exothermic).

1 |
Comparison of the distributions of H-bonds per water molecule along the interface.a Distribution of the average number of H-bonds per water molecule.b Distribution of the average length of H-bonds per water molecule.

Fig. 6 |
Schematics for two possible pathways of H2Oad deprotonation into a terminal hydroxyl OHt − and a H + in solution (step 2: H2Oad → OHt − + H + (sol)).Note: At the transition state (TS), the detaching H + bounds to the nearby water in solution.Next, the H-O bond of H2Oad breaks and forms the Zundel-like (H5O2 + ) structure with nearby water in the near interface.Then, the (H5O2 + ) structure through a series of catchand-abandon-like proton transfer chains forms a stable (H5O2 + ) structure in the liquid (FS1) or transfer the proton to the bridge oxygen site (Obr) on the surface (FS2).The proton in the Obr site would further detach and migrate to the liquid by a similar way of proton transfer.Supplementary Fig. 8 | Illustration of the stability of hexafluoroacetone during the photocatalytic process.Density of states (DOS) of TiO2(110) with OHt − and hexafluoroacetone adsorbed on the terminal Ti5c (a) and the corresponding geometry structure (b).c Energy profile of possibility of the hexafluoroacetone conversion assisted by the surface OH • radicals in the reaction condition.

Table 5 | Parameters of the water/TiO2(110) models using in the AIMD simulation at different temperatures (T= 298, 373, and 500
K).The density of water in different systems can be estimated by the formula: ρH2O =(N×MH2O)/ (NA×a×b×h), where N is the number of H2O molecules, corresponding to value of 26 in these systems; a and b is the length and width of the slab (a:11.84Å, b: 6.50 Å), and h is the actual thicknesses of the water structure.